Essential Role for FtsL in Activation of Septal Peptidoglycan Synthesis

Septal wall synthesis is sufficient to change ameba-like cells into uniform oval-shaped cells in Escherichia coli L-forms

A cell wall is required to control cell shape and size to maintain growth and division. However, some bacterial species maintain their morphology and size without a cell wall, calling into question the importance of the cell wall to maintain shape and size. It has been very difficult to examine the dispensability of cell wall synthesis in rod-shaped bacteria such as Escherichia coli for maintenance of their shape and size because they lyse without cell walls under normal culture conditions. Here, we show that wall-less E. coli L-form cells, which have a heterogeneous cell morphology, can be converted to a mostly uniform oval shape solely by FtsZ-dependent division, even in the absence of cylindrical cell wall synthesis. This FtsZ-dependent control of cell shape and size in the absence of a cell wall requires at least either the Min or nucleoid occlusion systems for positioning FtsZ at mid cell division sites.

Novel structural determinants and bacterial death-related regulatory effects of the scorpion defensin BmKDfsin4 against gram-positive bacteria

Numerous defensins constitute a family of cationic antimicrobial peptides with high degrees of sequence variability, and in-depth characterization of their structural basis and antibacterial mechanisms remains limited. Here, a representative scorpion defensin, BmKDfsin4, with two distinct hydrophobic and basic residue clusters, was extensively investigated. The hydrophobic residue cluster, formed by Phe2, Pro5, Phe6, Phe28 and Leu29 residues, strongly influences the antibacterial activity of BmKDfsin4 against Gram-positive bacteria. Compared with the three scattered Lys13, Lys30 and Arg32 residues, the basic residue cluster, consisting of the Arg19, Arg20, Arg21 and Arg37 residues, played a less important role. The synergistic interaction between the basic residue cluster and Arg32 significantly affected BmKDfsin4 function. The bacterial growth inhibition by BmKDfsin4 was associated with regulating expression levels of cell division-related genes, cell wall synthesis-related genes and bacterial autolysis-related genes rather than destroying the bacterial cell membrane. The coincubation of BmKDfsin4 with the bacterial strains induced gradual changes in the bacterial surface from a rough and thin surface to a noticeably wrinkled surface together with abundant white spots and even complete cavities within the bacteria. These findings revealed novel structural determinants and bacterial death-related regulatory effects of the defensin BmKDfsin4 and highlighted diverse antibacterial mechanisms of defensins.

A unique cell division protein critical for the assembly of the bacterial divisome

Identification of unique essential bacterial genes is important for not only the understanding of their cell biology but also the development of new antimicrobials. Here, we report a previously unrecognized core component of the Acinetobacter baumannii divisome. Our results reveal that the protein, termed Aeg1 interacts with multiple cell division proteins, including FtsN, which is required for components of the divisome to localize to the midcell. We demonstrate that the FtsAE202K and FtsBE65A mutants effectively bypassed the need of Aeg1 by A. baumannii, as did the activation variants FtsWM254I and FtsWS274G. Our results suggest that Aeg1 is a cell division protein that arrives at the division site to initiate cell division by recruiting FtsN, which activates FtsQLB and FtsA to induce the septal peptidoglycan synthase FtsWI. The discovery of the new essential cell division protein has provided a new target for the development of antibacterial agents.

The divisome is a self-enhancing machine in Escherichia coli and Caulobacter crescentus

During bacterial cytokinesis, polymers of the bacterial tubulin FtsZ coalesce into the Z ring to orchestrate divisome assembly and septal cell wall synthesis. Previous studies have found that Z ring condensation and stability is critical for successful cell division. However, how FtsZ filaments condense into a Z ring remains enigmatic and whether septal cell wall synthesis can feedback to the Z ring has not been investigated. Here, we show that FtsZ-associated proteins (Zaps) play important roles in Z ring condensation and stability, and discover septal cell wall synthesis as a novel player for Z ring condensation and stabilization in Escherichia coli and Caulobacter crescentus. Moreover, we find that the interaction between the Z ring membrane anchor, FtsA, and components of the septal cell wall synthetic complex are critical for septal cell wall synthesis-mediated Z ring condensation. Altogether, these findings suggest that the divisome is a self-enhancing machine in these two gram-negative bacteria, where the Z ring and the septal cell wall synthetic complex communicate with and reinforce each other to ensure robustness of cell division.

E. coli FtsN coordinates synthesis and degradation of septal peptidoglycan by partitioning between a synthesis track and a denuded glycan track

The E. coli cell division protein FtsN was proposed to coordinate septal peptidoglycan (sPG) synthesis and degradation to ensure robust cell wall constriction without lethal lesions. Although the precise mechanism remains unclear, previous work highlights the importance of two FtsN domains: the E domain, which interacts with and activates the sPG synthesis complex FtsWIQLB, and the SPOR domain, which binds to denuded glycan (dnG) strands, key intermediates in sPG degradation. Here, we used single-molecule tracking of FtsN and FtsW (a proxy for the sPG synthesis complex FtsWIQLB) to investigate how FtsN coordinates the two opposing processes. We observed dynamic behaviors indicating that FtsN's SPOR domain binds to dnGs cooperatively, which both sequesters the sPG synthesis complex on dnG (termed as the dnG-track) and protects dnGs from degradation by lytic transglycosylases (LTs). The release of the SPOR domain from dnGs leads to activating the sPG synthesis complex on the sPG-track and simultaneously exposing those same dnGs to degradation. Furthermore, FtsN's SPOR domain self-interacts and facilitates the formation of a multimeric sPG synthesis complex on both tracks. The cooperative self-interaction of the SPOR domain creates a sensitive switch to regulate the partitioning of FtsN between the dnG- and sPG-tracks, thereby controlling the balance between sequestered and active populations of the sPG synthesis complex. As such, FtsN coordinates sPG synthesis and degradation in space and time.

Role of the antiparallel double-stranded filament form of FtsA in activating the Escherichia coli divisome

The actin-like FtsA protein is essential for function of the cell division machinery, or divisome, in many bacteria including Escherichia coli. Previous in vitro studies demonstrated that purified wild-type FtsA assembles into closed mini-rings on lipid membranes, but oligomeric variants of FtsA such as FtsAR286W and FtsAG50E can bypass certain divisome defects and form arc and double-stranded (DS) oligomeric states, respectively, which may reflect conversion of an inactive to an active form of FtsA. However, it remains unproven which oligomeric forms of FtsA are responsible for assembling and activating the divisome. Here, we used an in vivo crosslinking assay for FtsA DS filaments to show that they largely depend on proper divisome assembly and are prevalent at later stages of cell division. We also used a previously reported variant that fails to assemble DS filaments, FtsAM96E R153D, to investigate the roles of FtsA oligomeric states in divisome assembly and activation. We show that FtsAM96E R153D cannot form DS filaments in vivo, fails to replace native FtsA, and confers a dominant negative phenotype, underscoring the importance of the DS filament stage for FtsA function. Surprisingly, however, activation of the divisome through the ftsL* or ftsW* superfission alleles suppressed the dominant negative phenotype and rescued the functionality of FtsAM96E R153D. Our results suggest that FtsA DS filaments are needed for divisome activation once it is assembled, but they are not essential for divisome assembly or guiding septum synthesis.IMPORTANCECell division is fundamental for cellular duplication. In simple cells like Escherichia coli bacteria, the actin homolog FtsA is essential for cell division and assembles into a variety of protein filaments at the cytoplasmic membrane. These filaments not only help tether polymers of the tubulin-like FtsZ to the membrane at early stages of cell division but also play crucial roles in recruiting other cell division proteins to a complex called the divisome. Once assembled, the E. coli divisome subsequently activates synthesis of the division septum that splits the cell in two. One recently discovered oligomeric conformation of FtsA is an antiparallel double-stranded filament. Using a combination of in vivo crosslinking and genetics, we provide evidence suggesting that these FtsA double filaments have a crucial role in activating the septum synthesis enzymes.

Role of the antiparallel double-stranded filament form of FtsA in activating the Escherichia coli divisome

The actin-like FtsA protein is essential for function of the cell division machinery, or divisome, in many bacteria including Escherichia coli. Previous in vitro studies demonstrated that purified wild-type FtsA assembles into closed mini-rings on lipid membranes, but oligomeric variants of FtsA such as FtsAR286W and FtsAG50E can bypass certain divisome defects and form arc and double-stranded (DS) oligomeric states, respectively, which may reflect conversion of an inactive to an active form of FtsA. Yet, it remains unproven which oligomeric forms of FtsA are responsible for assembling and activating the divisome. Here we used an in vivo crosslinking assay for FtsA DS filaments to show that they largely depend on proper divisome assembly and are prevalent at later stages of cell division. We also used a previously reported variant that fails to assemble DS filaments, FtsAM96E R153D, to investigate the roles of FtsA oligomeric states in divisome assembly and activation. We show that FtsAM96E R153D cannot form DS filaments in vivo, fails to replace native FtsA, and confers a dominant negative phenotype, underscoring the importance of the DS filament stage for FtsA function. Surprisingly, however, activation of the divisome through the ftsL* or ftsW* superfission alleles suppressed the dominant negative phenotype and rescued the functionallity of FtsAM96E R153D. Our results suggest that FtsA DS filaments are needed for divisome activation once it is assembled, but they are not essential for divisome assembly or guiding septum synthesis.

Integration of cell wall synthesis and chromosome segregation during cell division in Caulobacter

To divide, bacteria must synthesize their peptidoglycan (PG) cell wall, a protective meshwork that maintains cell shape. FtsZ, a tubulin homolog, dynamically assembles into a midcell band, recruiting division proteins, including the PG synthases FtsW and FtsI. FtsWI are activated to synthesize PG and drive constriction at the appropriate time and place. However, their activation pathway remains unresolved. In Caulobacter crescentus, FtsWI activity requires FzlA, an essential FtsZ-binding protein. Through time-lapse imaging and single-molecule tracking of Caulobacter FtsW and FzlA, we demonstrate that FzlA is a limiting constriction activation factor that signals to promote conversion of inactive FtsW to an active, slow-moving state. We find that FzlA interacts with the DNA translocase FtsK and place FtsK genetically in a pathway with FzlA and FtsWI. Misregulation of the FzlA-FtsK-FtsWI pathway leads to heightened DNA damage and cell death. We propose that FzlA integrates the FtsZ ring, chromosome segregation, and PG synthesis to ensure robust and timely constriction during Caulobacter division.

Insights into the assembly and regulation of the bacterial divisome

The ability to split one cell into two is fundamental to all life, and many bacteria can accomplish this feat several times per hour with high accuracy. Most bacteria call on an ancient homologue of tubulin, called FtsZ, to localize and organize the cell division machinery, the divisome, into a ring-like structure at the cell midpoint. The divisome includes numerous other proteins, often including an actin homologue (FtsA), that interact with each other at the cytoplasmic membrane. Once assembled, the protein complexes that comprise the dynamic divisome coordinate membrane constriction with synthesis of a division septum, but only after overcoming checkpoints mediated by specialized protein-protein interactions. In this Review, we summarize the most recent evidence showing how the divisome proteins of Escherichia coli assemble at the cell midpoint, interact with each other and regulate activation of septum synthesis. We also briefly discuss the potential of divisome proteins as novel antibiotic targets.

Building the Bacterial Divisome at the Septum

Across living organisms, division is necessary for cell survival and passing heritable information to the next generation. For this reason, cell division is highly conserved among eukaryotes and prokaryotes. Among the most highly conserved cell division proteins in eukaryotes are tubulin and actin. Tubulin polymerizes to form microtubules, which assemble into cytoskeletal structures in eukaryotes, such as the mitotic spindle that pulls chromatids apart during mitosis. Actin polymerizes to form a morphological framework for the eukaryotic cell, or cytoskeleton, that undergoes reorganization during mitosis. In prokaryotes, two of the most highly conserved cell division proteins are the tubulin homolog FtsZ and the actin homolog FtsA. In this chapter, the functions of the essential bacterial cell division proteins FtsZ and FtsA and their roles in assembly of the divisome at the septum, the site of cell division, will be discussed. In most bacteria, including Escherichia coli, the tubulin homolog FtsZ polymerizes at midcell, and this step is crucial for recruitment of many other proteins to the division site. For this reason, both FtsZ abundance and polymerization are tightly regulated by a variety of proteins. The actin-like FtsA protein polymerizes and tethers FtsZ polymers to the cytoplasmic membrane. Additionally, FtsA interacts with later stage cell division proteins, which are essential for division and for building the new cell wall at the septum. Recent studies have investigated how actin-like polymerization of FtsA on the lipid membrane may impact division, and we will discuss this and other ways that division in bacteria is regulated through FtsZ and FtsA.

Benzodioxane-benzamides as promising inhibitors of Escherichia coli FtsZ

The conserved process of cell division in bacteria has been a long-standing target for antimicrobials, although there are few examples of potent broad-spectrum compounds that inhibit this process. Most currently available compounds acting on division are directed towards the FtsZ protein, a self-assembling GTPase that is a central element of the division machinery in most bacteria. Benzodioxane-benzamides are promising candidates, but poorly explored in Gram-negatives. We have tested a number of these compounds on E. coli FtsZ and found that many of them significantly stabilized the polymers against disassembly and reduced the GTPase activity. Reconstitution in crowded cell-like conditions showed that FtsZ bundles were also susceptible to these compounds, including some compounds that were inactive on protofilaments in dilute conditions. They efficiently killed E. coli cells defective in the AcrAB efflux pump. The activity of the compounds on cell growth and division generally showed a good correlation with their effect in vitro, and our experiments are consistent with FtsZ being the target in vivo. Our results uncover the detrimental effects of benzodioxane-benzamides on permeable E. coli cells via its central division protein, implying that lead compounds may be found within this class for the development of antibiotics against Gram-negative bacteria.

Plasticity in the cell division processes of obligate intracellular bacteria

Most bacteria divide through a highly conserved process called binary fission, in which there is symmetric growth of daughter cells and the synthesis of peptidoglycan at the mid-cell to enable cytokinesis. During this process, the parental cell replicates its chromosomal DNA and segregates replicated chromosomes into the daughter cells. The mechanisms that regulate binary fission have been extensively studied in several model organisms, including Eschericia coli, Bacillus subtilis, and Caulobacter crescentus. These analyses have revealed that a multi-protein complex called the divisome forms at the mid-cell to enable peptidoglycan synthesis and septation during division. In addition, rod-shaped bacteria form a multi-protein complex called the elongasome that drives sidewall peptidoglycan synthesis necessary for the maintenance of rod shape and the lengthening of the cell prior to division. In adapting to their intracellular niche, the obligate intracellular bacteria discussed here have eliminated one to several of the divisome gene products essential for binary fission in E. coli. In addition, genes that encode components of the elongasome, which were mostly lost as rod-shaped bacteria evolved into coccoid organisms, have been retained during the reductive evolutionary process that some coccoid obligate intracellular bacteria have undergone. Although the precise molecular mechanisms that regulate the division of obligate intracellular bacteria remain undefined, the studies summarized here indicate that obligate intracellular bacteria exhibit remarkable plasticity in their cell division processes.

Conformational changes in the essential E. coli septal cell wall synthesis complex suggest an activation mechanism

The bacterial divisome is a macromolecular machine composed of more than 30 proteins that controls cell wall constriction during division. Here, we present a model of the structure and dynamics of the core complex of the E. coli divisome, supported by a combination of structure prediction, molecular dynamics simulation, single-molecule imaging, and mutagenesis. We focus on the septal cell wall synthase complex formed by FtsW and FtsI, and its regulators FtsQ, FtsL, FtsB, and FtsN. The results indicate extensive interactions in four regions in the periplasmic domains of the complex. FtsQ, FtsL, and FtsB support FtsI in an extended conformation, with the FtsI transpeptidase domain lifted away from the membrane through interactions among the C-terminal domains. FtsN binds between FtsI and FtsL in a region rich in residues with superfission (activating) and dominant negative (inhibitory) mutations. Mutagenesis experiments and simulations suggest that the essential domain of FtsN links FtsI and FtsL together, potentially modulating interactions between the anchor-loop of FtsI and the putative catalytic cavity of FtsW, thus suggesting a mechanism of how FtsN activates the cell wall synthesis activities of FtsW and FtsI.

The divisome but not the elongasome organizes capsule synthesis in Streptococcus pneumoniae

The bacterial cell envelope consists of multiple layers, including the peptidoglycan cell wall, one or two membranes, and often an external layer composed of capsular polysaccharides (CPS) or other components. How the synthesis of all these layers is precisely coordinated remains unclear. Here, we identify a mechanism that coordinates the synthesis of CPS and peptidoglycan in Streptococcus pneumoniae. We show that CPS synthesis initiates from the division septum and propagates along the long axis of the cell, organized by the tyrosine kinase system CpsCD. CpsC and the rest of the CPS synthesis complex are recruited to the septum by proteins associated with the divisome (a complex involved in septal peptidoglycan synthesis) but not the elongasome (involved in peripheral peptidoglycan synthesis). Assembly of the CPS complex starts with CpsCD, then CpsA and CpsH, the glycosyltransferases, and finally CpsJ. Remarkably, targeting CpsC to the cell pole is sufficient to reposition CPS synthesis, leading to diplococci that lack CPS at the septum. We propose that septal CPS synthesis is important for chain formation and complement evasion, thereby promoting bacterial survival inside the host.

Cryo-EM structure of the bacterial divisome core complex and antibiotic target FtsWIQBL

In most bacteria, cell division relies on the synthesis of new cell wall material by the multiprotein divisome complex. Thus, at the core of the divisome are the transglycosylase FtsW, which synthesises peptidoglycan strands from its substrate Lipid II, and the transpeptidase FtsI that cross-links these strands to form a mesh, shaping and protecting the bacterial cell. The FtsQ-FtsB-FtsL trimeric complex interacts with the FtsWI complex and is involved in regulating its enzymatic activities; however, the structure of this pentameric complex is unknown. Here, we present the cryogenic electron microscopy structure of the FtsWIQBL complex from Pseudomonas aeruginosa at 3.7 Å resolution. Our work reveals intricate structural details, including an extended coiled coil formed by FtsL and FtsB and the periplasmic interaction site between FtsL and FtsI. Our structure explains the consequences of previously reported mutations and we postulate a possible activation mechanism involving a large conformational change in the periplasmic domain. As FtsWIQBL is central to the divisome, our structure is foundational for the design of future experiments elucidating the precise mechanism of bacterial cell division, an important antibiotic target.

Structure of the heterotrimeric membrane protein complex FtsB-FtsL-FtsQ of the bacterial divisome

The synthesis of the cell-wall peptidoglycan during bacterial cell division is mediated by a multiprotein machine, called the divisome. The essential membrane protein complex of FtsB, FtsL and FtsQ (FtsBLQ) is at the heart of the divisome assembly cascade in Escherichia coli. This complex regulates the transglycosylation and transpeptidation activities of the FtsW-FtsI complex and PBP1b via coordination with FtsN, the trigger for the onset of constriction. Yet the underlying mechanism of FtsBLQ-mediated regulation is largely unknown. Here, we report the full-length structure of the heterotrimeric FtsBLQ complex, which reveals a V-shaped architecture in a tilted orientation. Such a conformation could be strengthened by the transmembrane and the coiled-coil domains of the FtsBL heterodimer, as well as an extended β-sheet of the C-terminal interaction site involving all three proteins. This trimeric structure may also facilitate interactions with other divisome proteins in an allosteric manner. These results lead us to propose a structure-based model that delineates the mechanism of the regulation of peptidoglycan synthases by the FtsBLQ complex.

New biochemistry in the Rhodanese-phosphatase superfamily: emerging roles in diverse metabolic processes, nucleic acid modifications, and biological conflicts

The protein-tyrosine/dual-specificity phosphatases and rhodanese domains constitute a sprawling superfamily of Rossmannoid domains that use a conserved active site with a cysteine to catalyze a range of phosphate-transfer, thiotransfer, selenotransfer and redox activities. While these enzymes have been extensively studied in the context of protein/lipid head group dephosphorylation and various thiotransfer reactions, their overall diversity and catalytic potential remain poorly understood. Using comparative genomics and sequence/structure analysis, we comprehensively investigate and develop a natural classification for this superfamily. As a result, we identified several novel clades, both those which retain the catalytic cysteine and those where a distinct active site has emerged in the same location (e.g. diphthine synthase-like methylases and RNA 2' OH ribosyl phosphate transferases). We also present evidence that the superfamily has a wider range of catalytic capabilities than previously known, including a set of parallel activities operating on various sugar/sugar alcohol groups in the context of NAD+-derivatives and RNA termini, and potential phosphate transfer activities involving sugars and nucleotides. We show that such activities are particularly expanded in the RapZ-C-DUF488-DUF4326 clade, defined here for the first time. Some enzymes from this clade are predicted to catalyze novel DNA-end processing activities as part of nucleic-acid-modifying systems that are likely to function in biological conflicts between viruses and their hosts.

Construction and Characterization of Functional FtsA Sandwich Fusions for Studies of FtsA Localization and Dynamics during Escherichia coli Cell Division

FtsA, a homolog of actin, is essential for cell division of Escherichia coli and is widely conserved among many bacteria. FtsA helps to tether polymers of the bacterial tubulin homolog FtsZ to the cytoplasmic membrane as part of the cytokinetic Z ring. GFP fusions to FtsA have illuminated FtsA's localization in live E. coli, but these fusions have not been fully functional and required the presence of the native FtsA. Here, we characterize "sandwich" fusions of E. coli FtsA to either mCherry or msfGFP that are functional for cell division and exhibit fluorescent rings at midcell that persist throughout constriction until cell separation. FtsA within the Z ring moved circumferentially like FtsZ, and FtsA outside the rings formed highly dynamic patches at the membrane. Notably, both FtsA-mCherry^sw and FtsA-msfGFP^sw acted as mild hypermorphs, as they were not toxic when overproduced, bypassed the essential cell division protein ZipA, and suppressed several thermosensitive fts alleles, although not as effectively as the prototypical hypermorph FtsA*. Overall, our results indicate that fluorescent FtsA sandwich fusions can be used as the sole FtsA in E. coli and thus should shed new light on FtsA dynamics during the cell division cycle in this model system. IMPORTANCE FtsA is a key conserved cell division protein, and E. coli is the most well studied model system for bacterial cell division. One obstacle to full understanding of this process is the lack of a fully functional fluorescent reporter for FtsA in vivo. Here, we describe a fluorescent fusion to E. coli FtsA that promotes efficient cell division in the absence of the native FtsA and can be used to monitor FtsA dynamics during cell division.

Cell wall synthesis and remodelling dynamics determine division site architecture and cell shape in Escherichia coli

The bacterial division apparatus catalyses the synthesis and remodelling of septal peptidoglycan (sPG) to build the cell wall layer that fortifies the daughter cell poles. Understanding of this essential process has been limited by the lack of native three-dimensional views of developing septa. Here, we apply state-of-the-art cryogenic electron tomography (cryo-ET) and fluorescence microscopy to visualize the division site architecture and sPG biogenesis dynamics of the Gram-negative bacterium Escherichia coli. We identify a wedge-like sPG structure that fortifies the ingrowing septum. Experiments with strains defective in sPG biogenesis revealed that the septal architecture and mode of division can be modified to more closely resemble that of other Gram-negative (Caulobacter crescentus) or Gram-positive (Staphylococcus aureus) bacteria, suggesting that a conserved mechanism underlies the formation of different septal morphologies. Finally, analysis of mutants impaired in amidase activation (ΔenvC ΔnlpD) showed that cell wall remodelling affects the placement and stability of the cytokinetic ring. Taken together, our results support a model in which competition between the cell elongation and division machineries determines the shape of cell constrictions and the poles they form. They also highlight how the activity of the division system can be modulated to help generate the diverse array of shapes observed in the bacterial domain.

FtsN maintains active septal cell wall synthesis by forming a processive complex with the septum-specific peptidoglycan synthases in E. coli

FtsN plays an essential role in promoting the inward synthesis of septal peptidoglycan (sPG) by the FtsWI complex during bacterial cell division. How it achieves this role is unclear. Here we use single-molecule tracking to investigate FtsN's dynamics during sPG synthesis in E. coli. We show that septal FtsN molecules move processively at ~9 nm s-1, the same as FtsWI molecules engaged in sPG synthesis (termed sPG-track), but much slower than the ~30 nm s-1 speed of inactive FtsWI molecules coupled to FtsZ's treadmilling dynamics (termed FtsZ-track). Importantly, processive movement of FtsN is exclusively coupled to sPG synthesis and is required to maintain active sPG synthesis by FtsWI. Our findings indicate that FtsN is part of the FtsWI sPG synthesis complex, and that while FtsN is often described as a "trigger" for the initiation for cell wall constriction, it must remain part of the processive FtsWI complex to maintain sPG synthesis activity.

MraZ Transcriptionally Controls the Critical Level of FtsL Required for Focusing Z-Rings and Kickstarting Septation in Bacillus subtilis

The bacterial division and cell wall (dcw) cluster is a highly conserved region of the genome which encodes several essential cell division factors, including the central divisome protein FtsZ. Understanding the regulation of this region is key to our overall understanding of the division process. mraZ is found at the 5' end of the dcw cluster, and previous studies have described MraZ as a sequence-specific DNA binding protein. In this article, we investigate MraZ to elucidate its role in Bacillus subtilis. Through our investigation, we demonstrate that increased levels of MraZ result in lethal filamentation due to repression of its own operon (mraZ-mraW-ftsL-pbpB). We observed rescue of filamentation upon decoupling ftsL expression, but not other genes in the operon, from MraZ control. Our data suggest that regulation of the mra operon may be an alternative way for cells to quickly arrest cytokinesis, potentially during entry into the stationary phase and in the event of DNA replication arrest. Furthermore, through time-lapse microscopy, we were able to identify that overexpression of mraZ or depletion of FtsL results in decondensation of the FtsZ ring (Z-ring). Using fluorescent d-amino acid labeling, we also observed that coordinated peptidoglycan insertion at the division site is dysregulated in the absence of FtsL. Thus, we reveal that the precise role of FtsL is in Z-ring maturation and focusing septal peptidoglycan synthesis. IMPORTANCE MraZ is a highly conserved protein found in a diverse range of bacteria, including genome-reduced Mycoplasma. We investigated the role of MraZ in Bacillus subtilis and found that overproduction of MraZ is toxic due to cell division inhibition. Upon further analysis, we observed that MraZ is a repressor of its own operon, which includes genes that encode the essential cell division factors FtsL and PBP2B. We noted that decoupling of ftsL alone was sufficient to abolish MraZ-mediated cell division inhibition. Using time-lapse microscopy, we showed that under conditions where the FtsL level is depleted, the cell division machinery is unable to initiate cytokinesis. Thus, our results pinpoint that the precise role of FtsL is in concentrating septal cell wall synthesis to facilitate cell division.

Comparative Study of Bacterial SPOR Domains Identifies Functionally Important Differences in Glycan Binding Affinity

Bacterial SPOR domains target proteins to the divisome by binding septal peptidoglycan (PG) at sites where cell wall amidases have removed stem peptides. These PG structures are referred to as denuded glycans. Although all characterized SPOR domains bind denuded glycans, whether there are differences in affinity is not known. Here, we use isothermal titration calorimetry (ITC) to determine the relative PG glycan binding affinity (<i>K</i>_d) of four Escherichia coli SPOR domains and one Cytophaga hutchinsonii SPOR domain. We found that the <i>K</i>_d values ranged from approximately 1 μM for E. coli DamX^SPOR and <i>C. hutchinsonii</i> CHU2221^SPOR to about 10 μM for E. coli FtsN^SPOR. To investigate whether these differences in PG binding affinity are important for SPOR domain protein function, we constructed and characterized a set of DamX and FtsN "swap" proteins. As expected, all SPOR domain swap proteins localized to the division site, and, in the case of FtsN, all of the heterologous SPOR domains supported cell division. However, for DamX, only the high-affinity SPOR domain from CHU2221 supported normal function in cell division. In summary, different SPOR domains bind denuded PG glycans with different affinities, which appears to be important for the functions of some SPOR domain proteins (e.g., DamX) but not for the functions of others (e.g., FtsN). <b>IMPORTANCE</b> SPOR domain proteins are prominent components of the cell division apparatus in a wide variety of bacteria. The primary function of SPOR domains is targeting proteins to the division site, which they accomplish by binding to septal peptidoglycan. However, whether SPOR domains have any functions beyond septal targeting is unknown. Here, we show that SPOR domains vary in their PG binding affinities and that, at least in the case of the E. coli cell division protein DamX, having a high-affinity SPOR domain contributes to proper function.

The structural dynamics of full-length divisome transmembrane proteins FtsQ, FtsB, and FtsL in FtsQBL complex formation

FtsQBL is a transmembrane protein complex in the divisome of Escherichia coli that plays a critical role in regulating cell division. Although extensive efforts have been made to investigate the interactions between the three involved proteins, FtsQ, FtsB, and FtsL, the detailed interaction mechanism is still poorly understood. In this study, we used hydrogen-deuterium exchange mass spectrometry to investigate these full-length proteins and their complexes. We also dissected the structural dynamic changes and the related binding interfaces within the complexes. Our data revealed that FtsB and FtsL interact at both the periplasmic and transmembrane regions to form a stable complex. Furthermore, the periplasmic region of FtsB underwent significant conformational changes. With the help of computational modeling, our results suggest that FtsBL complexation may bring the respective constriction control domains (CCDs) in close proximity. We show that when FtsBL adopts a coiled-coil structure, the CCDs are fixed at a vertical position relative to the membrane surface; thus, this conformational change may be essential for FtsBL’s interaction with other divisome proteins. In the FtsQBL complex, intriguingly, we show only FtsB interacts with FtsQ at its C-terminal region, which stiffens a large area of the β-domain of FtsQ. Consistent with this, we found the connection between the α- and β-domains in FtsQ is also strengthened in the complex. Overall, the present study provides important experimental evidence detailing the local interactions between the full-length FtsB, FtsL, and FtsQ protein, as well as valuable insights into the roles of FtsQBL complexation in regulating divisome activity.

Traditional Chinese Medicine Tanreqing Targets Both Cell Division and Virulence in Staphylococcus aureus

Staphylococcus aureus has been recognized as an important human pathogen and poses a serious health threat worldwide. With the advent of antibiotic resistance, such as the increased number of methicillin-resistant Staphylococcus aureus (MRSA), there is an urgent need to develop new therapeutical agents. In this study, Chinese traditional medicine Tanreqing (TRQ) has been used as an alternative treating agent against MRSA and we aim to unravel the mode of action of TRQ underlying MRSA inhibition. TRQ treatment affected numerous gene expression as revealed by RNA-seq analysis. Meanwhile, TRQ targeted cell division to inhibit cell growth as shown by illumination microscopy. Besides, we confirmed that TRQ downregulates the expression of virulence factors such as hemolysin and autolysin. Finally, we used a murine model to demonstrate that TRQ efficiently reduces bacterial virulence. Altogether, we have proved TRQ formula to be an effective agent against S. aureus infections.

An Updated Model of the Divisome: Regulation of the Septal Peptidoglycan Synthesis Machinery by the Divisome

The synthesis of a peptidoglycan septum is a fundamental part of bacterial fission and is driven by a multiprotein dynamic complex called the divisome. FtsW and FtsI are essential proteins that synthesize the peptidoglycan septum and are controlled by the regulatory FtsBLQ subcomplex and the activator FtsN. However, their mode of regulation has not yet been uncovered in detail. Understanding this process in detail may enable the development of new compounds to combat the rise in antibiotic resistance. In this review, recent data on the regulation of septal peptidoglycan synthesis is summarized and discussed. Based on structural models and the collected data, multiple putative interactions within FtsWI and with regulators are uncovered. This elaborates on and supports an earlier proposed model that describes active and inactive conformations of the septal peptidoglycan synthesis complex that are stabilized by these interactions. Furthermore, a new model on the spatial organization of the newly synthesized peptidoglycan and the synthesis complex is presented. Overall, the updated model proposes a balance between several allosteric interactions that determine the state of septal peptidoglycan synthesis.

Assembly and architecture of Escherichia coli divisome proteins FtsA and FtsZ

During Escherichia coli cell division, an intracellular complex of cell division proteins known as the Z-ring assembles at midcell during early division and serves as the site of constriction. While the predominant protein in the Z-ring is the widely conserved tubulin homolog FtsZ, the actin homolog FtsA tethers the Z-ring scaffold to the cytoplasmic membrane by binding to FtsZ. While FtsZ is known to function as a dynamic, polymerized GTPase, the assembly state of its partner, FtsA, and the role of ATP are still unclear. We report that a substitution mutation in the FtsA ATP-binding site impairs ATP hydrolysis, phospholipid vesicle remodeling in vitro, and Z-ring assembly in vivo. We demonstrate by transmission electron microscopy and Förster Resonance Energy Transfer that a truncated FtsA variant, FtsA(ΔMTS) lacking a C-terminal membrane targeting sequence, self assembles into ATP-dependent filaments. These filaments coassemble with FtsZ polymers but are destabilized by unassembled FtsZ. These findings suggest a model wherein ATP binding drives FtsA polymerization and membrane remodeling at the lipid surface, and FtsA polymerization is coregulated with FtsZ polymerization. We conclude that the coordinated assembly of FtsZ and FtsA polymers may serve as a key checkpoint in division that triggers cell wall synthesis and division progression.

The coiled-coil domain of Escherichia coli FtsLB is a structurally detuned element critical for modulating its activation in bacterial cell division

The FtsLB complex is a key regulator of bacterial cell division, existing in either an off state or an on state, which supports the activation of septal peptidoglycan synthesis. In Escherichia coli, residues known to be critical for this activation are located in a region near the C-terminal end of the periplasmic coiled-coil domain of FtsLB, raising questions about the precise role of this conserved domain in the activation mechanism. Here, we investigate an unusual cluster of polar amino acids found within the core of the FtsLB coiled coil. We hypothesized that these amino acids likely reduce the structural stability of the domain and thus may be important for governing conformational changes. We found that mutating these positions to hydrophobic residues increased the thermal stability of FtsLB but caused cell division defects, suggesting that the coiled-coil domain is a "detuned" structural element. In addition, we identified suppressor mutations within the polar cluster, indicating that the precise identity of the polar amino acids is important for fine-tuning the structural balance between the off and on states. We propose a revised structural model of the tetrameric FtsLB (named the "Y-model") in which the periplasmic domain splits into a pair of coiled-coil branches. In this configuration, the hydrophilic terminal moieties of the polar amino acids remain more favorably exposed to water than in the original four-helix bundle model ("I-model"). We propose that a shift in this architecture, dependent on its marginal stability, is involved in activating the FtsLB complex and triggering septal cell wall reconstruction.

Identification of the potential active site of the septal peptidoglycan polymerase FtsW

SEDS (Shape, Elongation, Division and Sporulation) proteins are widely conserved peptidoglycan (PG) glycosyltransferases that form complexes with class B penicillin-binding proteins (bPBPs, with transpeptidase activity) to synthesize PG during bacterial cell growth and division. Because of their crucial roles in bacterial morphogenesis, SEDS proteins are one of the most promising targets for the development of new antibiotics. However, how SEDS proteins recognize their substrate lipid II, the building block of the PG layer, and polymerize it into glycan strands is still not clear. In this study, we isolated and characterized dominant-negative alleles of FtsW, a SEDS protein critical for septal PG synthesis during bacterial cytokinesis. Interestingly, most of the dominant-negative FtsW mutations reside in extracellular loops that are highly conserved in the SEDS family. Moreover, these mutations are scattered around a central cavity in a modeled FtsW structure, which has been proposed to be the active site of SEDS proteins. Consistent with this, we found that these mutations blocked septal PG synthesis but did not affect FtsW localization to the division site, interaction with its partners nor its substrate lipid II. Taken together, these results suggest that the residues corresponding to the dominant-negative mutations likely constitute the active site of FtsW, which may aid in the design of FtsW inhibitors.

SEDS (Shape, Elongation, Division and Sporulation) proteins are widely conserved peptidoglycan polymerases that play critical roles in cell elongation and cell division in rod-shaped bacteria. However, how they catalyze PG polymerization remains poorly understood. In this study, we isolated and characterized a set of dominant-negative mutations in the SEDS protein FtsW, which synthesizes septal peptidoglycan during cell division in most bacteria. Our results revealed that the dominant-negative mutations disrupt FtsW’s ability to synthesize peptidoglycan, but do not affect its other activities, suggesting that the corresponding amino acids may constitute the active site of FtsW.

Localization, Assembly, and Activation of the Escherichia coli Cell Division Machinery

Decades of research, much of it in Escherichia coli, have yielded a wealth of insight into bacterial cell division. Here, we provide an overview of the E. coli division machinery with an emphasis on recent findings. We begin with a short historical perspective into the discovery of FtsZ, the tubulin homolog that is essential for division in bacteria and archaea. We then discuss assembly of the divisome, an FtsZ-dependent multiprotein platform, at the midcell septal site. Not simply a scaffold, the dynamic properties of polymeric FtsZ ensure the efficient and uniform synthesis of septal peptidoglycan. Next, we describe the remodeling of the cell wall, invagination of the cell envelope, and disassembly of the division apparatus culminating in scission of the mother cell into two daughter cells. We conclude this review by highlighting some of the open questions in the cell division field, emphasizing that much remains to be discovered, even in an organism as extensively studied as E. coli.

Recruitment of the TolA protein to cell constriction sites in Escherichia coli via three separate mechanisms, and a critical role for FtsWI activity in recruitment of both TolA and TolQ

The Tol-Pal system of Gram-negative bacteria helps maintain integrity of the cell envelope and ensures that invagination of the envelope layers during cell fission occurs in a well-coordinated manner. In E. coli, the five Tol-Pal proteins (TolQ, R, A, B and Pal) accumulate at cell constriction sites in a manner that normally requires the activity of the cell constriction initiation protein FtsN. While septal recruitment of TolR, TolB and Pal also requires the presence of TolQ and/or TolA, each of the the latter two can recognize constriction sites independently of the other system proteins. What attracts TolQ or TolA to these sites is unclear. We show that FtsN attracts both proteins in an indirect fashion, and that PBP1A, PBP1B and CpoB are dispensable for their septal recruitment. However, the β-lactam aztreonam readily interferes with septal accumulation of both TolQ and TolA, indicating that FtsN-stimulated production of septal peptidoglycan by the FtsWI synthase is critical to their recruitment. We also discovered that each of TolA's three domains can recognize division sites in a separate fashion. Notably, the middle domain (TolAII) is responsible for directing TolA to constriction sites in the absence of other Tol-Pal proteins and CpoB, while recruitment of TolAI and TolAIII requires TolQ and a combination of TolB, Pal, and CpoB, respectively. Additionally, we describe the construction and use of functional fluorescent sandwich fusions of the ZipA division protein, which should be more broadly valuable in future studies of the E. coli cell division machinery. IMPORTANCE Cell division (cytokinesis) is a fundamental biological process that is incompletely understood for any organism. Division of bacterial cells relies on a ring-like machinery called the septal ring or divisome that assembles along the circumference of the mother cell at the site where constriction will eventually occur. In the well-studied bacterium Escherichia coli, this machinery contains over thirty distinct proteins. We studied how two such proteins, TolA and TolQ, which also play a role in maintaining integrity of the outer-membrane, are recruited to the machinery. We find that TolA can be recruited by three separate mechanisms, and that both proteins rely on the activity of a well-studied cell division enzyme for their recruitment.

FtsA acts through FtsW to promote cell wall synthesis during cell division in Escherichia coli

In Escherichia coli, FtsQLB is required to recruit the essential septal peptidoglycan (sPG) synthase FtsWI to FtsA, which tethers FtsZ filaments to the membrane. The arrival of FtsN switches FtsQLB in the periplasm and FtsA in the cytoplasm from a recruitment role to active forms that synergize to activate FtsWI. Genetic evidence indicates that the active form of FtsQLB has an altered conformation with an exposed domain of FtsL that acts on FtsI to activate FtsW. However, how FtsA contributes to the activation of FtsW is not clear, as it could promote the conformational change in FtsQLB or act directly on FtsW. Here, we show that the overexpression of an activated FtsA (FtsA*) bypasses FtsQ, indicating it can compensate for FtsQ's recruitment function. Consistent with this, FtsA* also rescued FtsL and FtsB mutants deficient in FtsW recruitment. FtsA* also rescued an FtsL mutant unable to deliver the periplasmic signal from FtsN, consistent with FtsA* acting on FtsW. In support of this, an FtsW mutant was isolated that was rescued by an activated FtsQLB but not by FtsA*, indicating it was specifically defective in activation by FtsA. Our results suggest that in response to FtsN, the active form of FtsA acts on FtsW in the cytoplasm and synergizes with the active form of FtsQLB acting on FtsI in the periplasm to activate FtsWI to carry out sPG synthesis.

Growth and Division of the Peptidoglycan Matrix

Most bacteria are surrounded by a peptidoglycan cell wall that defines their shape and protects them from osmotic lysis. The expansion and division of this structure therefore plays an integral role in bacterial growth and division. Additionally, the biogenesis of the peptidoglycan layer is the target of many of our most effective antibiotics. Thus, a better understanding of how the cell wall is built will enable the development of new therapies to combat the rise of drug-resistant bacterial infections. This review covers recent advances in defining the mechanisms involved in assembling the peptidoglycan layer with an emphasis on discoveries related to the function and regulation of the cell elongation and division machineries in the model organisms Escherichia coli and Bacillus subtilis. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A two-track model for the spatiotemporal coordination of bacterial septal cell wall synthesis revealed by single-molecule imaging of FtsW

Synthesis of septal peptidoglycan (sPG) is crucial for bacterial cell division. FtsW, an indispensable component of the cell division machinery in all walled bacterial species, was recently identified in vitro as a peptidoglycan glycosyltransferase (PGTase). Despite its importance, the septal PGTase activity of FtsW has not been demonstrated in vivo. How its activity is spatiotemporally regulated in vivo has also remained elusive. Here, we confirmed FtsW as an essential septum-specific PGTase in vivo using an N-acetylmuramic acid analogue incorporation assay. Next, using single-molecule tracking coupled with genetic manipulations, we identified two populations of processively moving FtsW molecules: a fast-moving population correlated with the treadmilling dynamics of the essential cytoskeletal FtsZ protein and a slow-moving population dependent on active sPG synthesis. We further identified that FtsN, a potential sPG synthesis activator, plays an important role in promoting the slow-moving population. Our results suggest a two-track model, in which inactive sPG synthases follow the 'Z-track' to be distributed along the septum and FtsN promotes their release from the Z-track to become active in sPG synthesis on the slow 'sPG-track'. This model provides a mechanistic framework for the spatiotemporal coordination of sPG synthesis in bacterial cell division.

Genetic analysis of the septal peptidoglycan synthase FtsWI complex supports a conserved activation mechanism for SEDS-bPBP complexes

SEDS family peptidoglycan (PG) glycosyltransferases, RodA and FtsW, require their cognate transpeptidases PBP2 and FtsI (class B penicillin binding proteins) to synthesize PG along the cell cylinder and at the septum, respectively. The activities of these SEDS-bPBPs complexes are tightly regulated to ensure proper cell elongation and division. In Escherichia coli FtsN switches FtsA and FtsQLB to the active forms that synergize to stimulate FtsWI, but the exact mechanism is not well understood. Previously, we isolated an activation mutation in ftsW (M269I) that allows cell division with reduced FtsN function. To try to understand the basis for activation we isolated additional substitutions at this position and found that only the original substitution produced an active mutant whereas drastic changes resulted in an inactive mutant. In another approach we isolated suppressors of an inactive FtsL mutant and obtained FtsW^E289G and FtsI^K211I and found they bypassed FtsN. Epistatic analysis of these mutations and others confirmed that the FtsN-triggered activation signal goes from FtsQLB to FtsI to FtsW. Mapping these mutations, as well as others affecting the activity of FtsWI, on the RodA-PBP2 structure revealed they are located at the interaction interface between the extracellular loop 4 (ECL4) of FtsW and the pedestal domain of FtsI (PBP3). This supports a model in which the interaction between the ECL4 of SEDS proteins and the pedestal domain of their cognate bPBPs plays a critical role in the activation mechanism.

Bacterial cell division requires the synthesis of septal peptidoglycan by the widely conserved SEDS-bPBP protein complex FtsWI, but how the complex is activated during cell division is still poorly understood. Previous studies suggested that FtsN initiates a signaling cascade in the periplasm to activate FtsWI. Here we isolated and characterized activated FtsW and FtsI mutants and confirmed that the signaling cascade for FtsW activation goes from FtsN to FtsQLB to FtsI and then to FtsW. The residues corresponding to mutations affecting FtsWI activation are clustered to a small region of the interaction interface between the pedestal domain of FtsI and the extracellular loop 4 of FtsW, suggesting that this interaction mediates activation of FtsW. This is strikingly similar to the proposed activation mechanism for the RodA-PBP2 complex, another SEDS-bPBP complex required for cell elongation. Thus, the two homologous SEDS-bPBP complexes are activated similarly by completely unrelated activators that modulate the interaction interface between the SEDS proteins and the bPBPs.

Bioengineering of Bordetella pertussis Adenylate Cyclase Toxin for Vaccine Development and Other Biotechnological Purposes

The adenylate cyclase toxin, CyaA, is one of the key virulent factors produced by Bordetella pertussis, the causative agent of whooping cough. This toxin primarily targets innate immunity to facilitate bacterial colonization of the respiratory tract. CyaA exhibits several remarkable characteristics that have been exploited for various applications in vaccinology and other biotechnological purposes. CyaA has been engineered as a potent vaccine vehicle to deliver antigens into antigen-presenting cells, while the adenylate cyclase catalytic domain has been used to design a robust genetic assay for monitoring protein-protein interactions in bacteria. These two biotechnological applications are briefly summarized in this chapter.